It has been known that superior insulating construction materials such as tiles and wall boards can be achieved by a polycondensation process whereby carboxyl groups forming part of an olefinic base are linked with silanol groups contained in a silicone material. Additionally, the silanol groups are linked with hydroxyl groups contained in a hydrated fire retardant filler to provide an insulation composition that tolerates large inclusions of the filler material. Such a process is disclosed and described in European patent application publication No. EP 0 333 514 A1.
While the process claimed that the cross-linking had allowed for the inclusion of larger amounts of fire retardant filler materials into the insulative composition, it was soon discovered that these compositions could not be processed into wire insulation of commercial quality.
One of the drawbacks of the chemistry was that the compositions taught by the subject patent application could not be easily extruded.
Another difficulty with the new insulative compositions was their characteristically sub-standard commercial tensile strength, elongation and flexibility.
It was soon determined that the increased cross-linking of these compositions, while improving the fire retardancy by reason of providing for increased filler loadings, nonetheless was overly binding the polymer chains. Thus, flexibility and elongation were impaired. Such over-binding, while useful for tiles and wall construction materials, which are not commercially affected by the reduced flexibility and elongation, is not useful for wire and cable insulation applications.
The present invention reflects the discovery that commercial grade insulation for wire and cable, particularly plenum wire and cable and heat-shrinkable tubing, can be fabricated by appreciably reducing the cross-linking of the components of the polycondensation process.
The chemistry of the aforesaid patent application utilized olefinic base materials containing 6% carboxyl groups by weight. This amount of the carboxyl groups is too high a percentage for fabricating commercial grades of wire insulation.
The present invention has determined that the viable range of carboxyl groups should be approximately below 3% by weight and preferably about 1% by weight of the base material.
In addition, while the prior application stressed the need to include dialdehydes to promote the cross-linking of the compositional components, the present invention reflects the discovery that they are detrimental to the processing of the insulation extrudate. The dialdehydes generally increase the viscosity beyond workable extrusion limits. Also, the dialdehydes reduce the temperatures at which the extrusion can be accomplished, thus further increasing the working viscosity, and further impairing the processability of the resultant composition. Compositions containing the dialdehydes were found to cause the extrusion heads to rupture due to increased pressure and viscosity. The prior application teaches the necessity for the inclusion of the dialdehydes, but that is because the formulations set forth therein relate primarily to construction materials and not to wire insulation.
The prior patent application further teaches the inclusion of an ethylene propylene elastomer in wire insulation compositions. These terpolymer blends were illustrative of the preferred wire insulation compositions, but subsequent testing revealed that they provided wire insulation with only marginal physical properties.
The current invention features a true terpolymer system rather than a blended terpolymer system. The terpolymers of the invention comprise a carboxylated ethylene vinyl acetate, without the inclusion of the elastomer, in order to improve the flexibility and the amount of filler that can be introduced into the system.
In addition to the changed chemistry, the invention subjected the new compositions to electron beam irradiation in order to improve the tensile strength, and to allow generally higher loadings of the flame retardant filler materials.
Normally, when greater amounts of filler materials are employed, the tensile strength and elongation decrease. Irradiation often improves tensile strength, but has never been found to increase the elongation. The present invention has shown that without the silanol groupings, the resultant composition will not show an increase in elongation after irradiation, as expected.
However, this invention incorporates the discovery that there is a synergy between the new chemistry and the irradiation, wherein the combination of the two processes (i.e., new chemistry and irradiation) provides for both an increase in tensile strength and in elongation.
This dual increase is valid for greater than 200 parts by weight of the inclusion of a hydroxyl-containing filler material into the terpolymer system.
Thus, the present invention has produced a new flame retardant wire and cable insulation with higher loadings of hydrated flame retardant fillers beyond those limits depicted in the aforementioned patent application. The effect of this new insulation on flame retardancy is dramatic, wherein the oxygen index for the new formulations is about 200% greater than the best wire and cable polycondensation formula.
Furthermore, the present invention has introduced the higher filler loadings without impairing the physical characteristics of the composition, whereby commercial grade insulation for a non-halogenated wire and cable is now feasible for the first time at these higher filler loadings.
The present invention features a non-halogenated composition for wire and cable and heat-shrinkable tubing insulation having superior flame retardancy, with oxygen indices approaching 70.0 or better. The inventive insulation is further characterized both by increased tensile strength and by increased elongation after irradiation, a result heretofore unknown in the prior art.